This invention relates to an apparatus for estimating a neutral position for the steering handle of a motor vehicle. Although the invention will be described in connection with a rear wheel steering control system, it should be understood that the invention is equally applicable to other vehicle dynamic characteristic control systems including active suspension control systems and the like utilizing information on steering wheel position to control the vehicle dynamic characteristics including yaw velocity, side velocity, roll velocity, etc.
For example, Japanese Patent Kokai No. 59-26341 discloses a steering handle neutral position estimating apparatus which employs a steering handle position sensor for sensing the existing steering handle position. The existing steering handle position is set for a steering handle neutral position when two conditions are fulfilled; that is, when the existing steering handle position is iwthin a predetermined range, and when the vehicle has traveled from the rest at a distance greater than a reference value. The steering handle neutral position estimating apparatus can estimate the steering handle neutral position with high accuracy by increasing the reference value with increasing traveling distance. However, the steering handle neutral position is estimated sometimes based on an erroneous neutral position signal which may be produced when the vehicle turns at a low speed in a circle of small radius with the steering handle being rotated at 360.degree. or which may be produced when the vehicle turns in a circle of great radius at a high speed creating a great tire slip angle with the steering handle being rotated at a small angle.